1. Field of the Invention
The present invention relates generally to enhancing the performance of disk drives by controlling thermal rise and, more particularly, to improved systems and methods for preventing a coil of a voice coil motor (VCM) from overheating by preventing excessive current from being applied to the coil.
2. Background of the Invention
A typical hard disk drive includes a head disk assembly (HDA) and a printed circuit board assembly (PCBA). The HDA includes at least one magnetic disk (disk), a spindle motor for rotating the disk, and a head stack assembly (HSA) that includes a read/write head with at least one transducer for reading and/or writing data. The HSA is controllably positioned by a servo system to read or write information from or to particular tracks on the disk. The typical HSA has three primary portions: (1) an actuator assembly that moves in response to the servo control system; (2) a head gimbal assembly (HGA) that extends from the actuator assembly and biases the head toward the disk; and (3) a flex cable assembly that provides an electrical interconnect with minimal constraint on movement.
A xe2x80x9crotaryxe2x80x9d or xe2x80x9cswing-typexe2x80x9d actuator assembly comprises a body portion that rotates on a pivot bearing cartridge between limited positions, a coil portion that extends from one side of the body portion to interact with one or more permanent magnets to form a VCM, and an actuator arm that extends from an opposite side of the body portion to support the HGA.
Within the HDA, the spindle motor rotates the disk or disks, that are the media to and from which the data signals are transmitted via the read write/head(s) on the gimbal attached to the load beam. The performance of the disk drive is largely dominated by its mechanical latencies. One such mechanical latency is the rotational latency of the drive, which is a function of rotational speed of the disk and hence of the spindle motor. Another such mechanical latency is the seek latency of the drive, which is a function of the speed at which the actuator radially moves across the disk.
Competitive pressures in the disk drive market have compelled disk drive designers and manufacturers to simultaneously boost performance and reduce cost. Historically, higher performance has been achieved by, for example, increasing the rotational speed of the spindle motor and/or performing faster seek operations. Faster seek operations, in turn, can be achieved by increasing the control current flowing through the VCM, thereby increasing the actuator""s acceleration and deceleration as it moves across the disk. Excessive VCM control currents or control current profiles having a high average value, however, can cause the VCM assembly (typically overmolded with a plastic material) to overheat, causing damage to the coil and the drive. For example, when subjected to an instantaneous or average current that is beyond the VCM""s design limitations, the coil can generate excessive heat with consequences such as delamination of the coil overmold material, or loss of rigidity, thus drooping and contacting adjacent magnets; and/or outgassing particulates into the disk drive enclosure, with deleterious results. Such outgassing from the coil overmold, coil insulators, and/or from other materials applied to the coil wires (such as wire insulators, for example) can occur even at relatively low temperatures (85xc2x0 C., for example). A need, therefore, exists to monitor the temperature of the VCM coil and to prevent damage thereto.
One possible solution that addresses the need to prevent excessive VCM temperatures is to limit the VCM control current so that the heat generated therein remains at all times within conservative limits, independent of present actuator current usage patterns. This solution, while effectively preventing the VCM from overheating and obviating the need to monitor the temperature thereof, also results in unacceptably slow drive performance. Another solution is proposed in the U.S. Pat. No. 5,594,603, issued to Mori et al. In the Mori patent, the current applied to the VCM is used to calculate an approximation of the VCM temperature. This method attempts to mathematically model the thermal behavior of the VCM by devising a number of coefficients and by quantifying and inter-relating the VCM control current, the heat naturally radiated by the VCM, the ambient temperature, the thermal capacity of the VCM, and the ambient temperature thereof, among other factors. However, such a mathematical model, although providing an indication of the present VCM temperature, may not accurately provide a calculated temperature value that accords with the present and actual temperature of the VCM. Indeed, a number of factors can skew the results obtained from such mathematical models. For example, the present temperature of the drive or the resistance of the VCM coil may not remain constant and result in changing VCM control current magnitudes. As the VCM control current is used as the basis for the temperature calculations, the VCM is not driven (i.e., supplied with VCM control current) in an optimal manner and the actuator may not sweep as rapidly across the disk as it might otherwise have, thereby needlessly limiting the overall performance of the drive. Alternatively, should the mathematical model prove to be an inaccurate predictor of actual VCM temperature in certain situations, excessive VCM control currents can be generated, potentially causing damage to the VCM and to the drive. Over many iterations, recursively-applied mathematical models can cause a relatively small error in each calculation to grow to such a degree that the model no longer accurately reflects present operating conditions. Reliance upon such an inexact mathematical model in modulating the VCM control current can understandably result in less than optimal drive performance characteristics.
Another proposed solution is proposed in the U.S. Pat. No. 5,128,813, issued to Lee. In this patent, a discrete temperature-sensing element is used to dynamically sense the VCM temperature during the operation of the drive. The output of the temperature-sensing element (e.g., thermistor) is quantized and used to calculate a multiplication factor. The multiplication factor, in turn, is multiplied by a reference velocity command during a seek operation to produce a velocity command that then is compared with a feedback velocity value to generate an error signal that modulates the operation of the actuator (e.g., the VCM control current) during seek operations. This patent discloses that the thermistor is mounted for thermal conduction directly to the head and disk assembly. While the temperature sensing element can, in fact, provide a direct measurement of the temperature of the VCM (in contrast to the Mori patent above, for example), this method requires mounting a high precision thermistor to the HDA and requires that appropriate signal conditioning means be provided to measure, quantize and interpret the resistance thereof. In many aspects, however, disk drive designers and manufacturers operate in an environment that has acquired many of the characteristics of a commodity market. In such a market, the addition of even a single, inexpensive part can directly and adversely affect competitiveness. In this case, therefore, the addition of the thermistor and associated signal conditioning means discussed in the Lee patent would be of little practical value.
Other proposed solutions to prevent a coil of a VCM from overheating have included the addition of a dwell time between successive seek operations. By adding the dwell time, no current is applied to the coil for some period after each seek operation. As a result, the disk drive permits the coil to cool during the dwell time; however, no further seek operations can be commenced until the dwell time ends. Thereby, although the coil is provided with an opportunity to cool, the performance of the drive is adversely affected by increasing the average seek time.
Similarly, it has been proposed that the temperature of the coil can be controlled by selecting a fixed maximum current for all seek distances exceeding a certain seek distance and then adjusting the acceleration and deceleration intervals during which the fixed maximum current is applied to the coil. The fixed maximum current is applied to all seek distances over the certain seek distance without regard to the existence of a coast interval. The coast interval is a time period that occurs between the acceleration and deceleration intervals. At the end of the acceleration interval, the head has reached a maximum velocity, and the fixed maximum current is removed from the coil. The head then effectively xe2x80x9ccoastsxe2x80x9d until the beginning of the deceleration interval when the fixed maximum current again is applied to the coil, but in an opposite direction, to decelerate the head. Since no current is applied to the coil as the head coasts, the coil is permitted to cool during the coast interval. As the coast interval increases with longer seek distances, the time during which the coil cools also increases. The proposed solution that uses fixed maximum current however does not take advantage of the increased cooling provided by the coast interval. Due to the increased cooling for the longer seek distances, the current applied to the coil for the longer seek distances can exceed the fixed maximum current, increasing the performance of the disk drive without causing the coil to overheat.
What are needed, therefore, are methods for preventing the application of excessive VCM control currents to a disk drive voice coil motor that are accurate, reliable and inexpensive in their implementation. More specifically, without relying upon complex and error prone mathematical modeling schemes or upon costly temperature sensing circuitry, methods for optimizing a maximum VCM control current to be applied to the voice coil motor for preselected seek distances are needed. Further, methods are needed for allowing the VCM control current to be modulated in an optimal manner to optimize seek operations.
The present invention is directed to a disk drive that provides the capability to prevent a coil of a voice coil motor from overheating due to the application of excessive current while moving a head over a recording surface of a disk.
A disk drive in accordance with an embodiment of the present invention comprises a disk with a recording surface, a head for reading and/or writing data on the recording surface, a voice coil motor for moving the head over the recording surface, and a servo control system. The servo control system applies a current to a coil of the voice coil motor, causing the head to move a seek distance over the recording surface of the disk. The servo control system generates a plurality of seek profiles for each of a plurality of seek distances and a plurality of current limits for the plurality of seek profiles. Each of the plurality of seek profiles defines a plan for controlling the current to be applied to the coil while the voice coil motor is operated over the seek distance. The plurality of current limits each define a maximum current allowed while controlling the current to be applied to the coil.
Each of the plurality of current limits is determined by examining a seek distance that represents one seek distance or a range of seek distances. For the seek distance, an appropriate seek profile and a nominal maximum current level are selected. The seek profile provides, among other things, time intervals during which the current is applied to the coil for the seek distance. The time intervals of the seek profile include an acceleration interval and a deceleration interval. If the seek distance exceeds a certain threshold length, typically thirty-five percent of full stroke, the seek profile also includes a coast interval, during which no current is applied to the coil. The nominal maximum current level comprises a starting point for determining the calculating the current limit for the seek distance and includes a nominal maximum acceleration current level and a nominal maximum deceleration current level.
A maximum stabilized RMS power for the seek distance then is calculated for the coil based upon several factors, including the acceleration interval, the deceleration interval, the nominal maximum acceleration current level, and the nominal maximum deceleration current level. If the maximum stabilized RMS power falls outside a preselected range of a maximum RMS power level for the coil, the nominal maximum acceleration current level and the nominal maximum deceleration current level each are adjusted, and the maximum stabilized RMS power is re-calculated. When the maximum stabilized RMS power is within the preselected range, the nominal maximum acceleration current level and the nominal maximum deceleration current level each, as adjusted, are stored as the current limit for the seek distance and, if desired, a next seek distance is examined. Once the current limit has been calculated for each of the plurality of seek distances, a current limit function, comprising the current limit for each seek distance, is generated. The current limit function may be generated in the form of a table, an equation, an algorithm, and/or any other form of generalized function, and, upon receiving a seek distance, produces a relevant current limit for the seek distance.
In operation, the servo control system receives a seek distance for moving the head over the recording surface. Upon receiving the seek distance, a relevant current limit, comprising a maximum acceleration current level and a maximum deceleration current level, is determined via the current limit function. The seek distance and the relevant current limit then are provided to a seek profile generator. In the seek profile generator, the relevant current limit is combined with a relevant seek profile, a seek profile from the plurality of seek profiles that is relevant to the seek distance. The relevant seek profile includes an acceleration interval, a deceleration interval, and, depending on the length of the seek distance, a coast interval. A current then is generated having a maximum amplitude substantially equal to the maximum acceleration current level during the acceleration interval and the maximum deceleration current level during the deceleration interval. No current is applied to the coil during the coast interval, if applicable. Once generated, the servo control system applies the current to the coil to move the head by the seek distance, maintaining the performance objectives of the disk drive but without exceeding the power handling capabilities of the coil.